n-Hexane Hydrogenolysis Behavior of Alumina-Supported Palladium–Platinum Alloys

Abstract: The behavior of alumina-supported palladium-platinum catalysts in the reaction of n-hexane hydrogenolysis was analyzed. The monometallic Pt/Al₂O₃ catalyst showed much higher activity than the rest of catalysts. The relationship between the catalytic activity of Pd–Pt/Al₂O₃ catalysts and bulk Pd–Pt composition matched the accepted relation between the surface composition and bulk composition of Pd–Pt, confirming a high surface enrichment in palladium. A linear relation between the apparent activation energy and preexponential factor (compensation effect) suggests that the reaction mechanism is similar for all Pd–Pt catalysts. However, after very high temperature of catalyst reduction (at 600 °C), the behavior of monometallic Pd/Al₂O₃ catalyst exhibited a considerable departure from the compensation plot, and drastic variations in C₁–C₅ products distribution. Such big changes are rationalized by assuming the transformation of Pd/Al₂O₃ into Pd–Al alloy, occurring at very high temperature of reduction. Product distribution of n-hexane hydrogenolysis for Pt/Al₂O₃ differs from that of Pd/Al₂O₃, showing more internal bond splitting than a terminal demethylation. However, bimetallic Pd–Pt/Al₂O₃ catalysts showed even more ‘internal bond splitting’ character than the Pt/Al₂O₃. Reasons for this synergism and changes in the behavior of Pd/Al₂O₃ catalysts are analyzed in terms of an electronic interaction between metals and alumina support. Graphic Abstract: [Figure not available: see fulltext.].